This invention relates in general to a tape driving apparatus and, particularly to a magnetic tape recording and/or reproducing apparatus, such as a video cassette recorder (VCR for short), and more particularly it relates to a mechanism for controlling the tape tension while the tape is running and winding around a reel.
Conventionally, a VCR has two reels on which hubs of a tape cassette are placed, and in this state a magnetic tape is drawn out of the cassette and it is bound about a cylinder. In any forward mode, such as a recording mode, a reproducing mode or a East forward mode, both reels rotate for supplying the tape from one of the reels, which is defined as a supply reel (S-reel), and simultaneously for winding the tape around the other reel which is defined as a take-up reel (T-reel). These names of the reels are not exchanged with each other even if operation occurs in a reverse mode.
When the tape is running in a normal direction, namely in the forward mode, torque of the S-reel is controlled by a brake mechanism which touchs the S-reel to regulate a rotational speed of the S-reel in response to the tape tension, so that the tape tension remains constant.
On the other hand, when the tape is running in the reverse direction, namely in the reverse mode, for example, in a reverse reproducing mode or a rewind mode, the brake mechanism is removed from controlling the torque of the S-reel, because it functions to not make the tape tension constant but instead to increase the deflection of the tape tension in the reverse mode. Instead a slip mechanism included in the S-reel regulates the reel torque to be constant. However, the tape tension is in inverse proportion to the winding radius which corresponds to the thickness of the tape which has been wound around the reel, so the tape tension changes about two or three times from the beginning to the end of being wound around S-reel. Therefore, although the torque of the S-reel is constant, the tape tension changes, and such change brings about various inconvenient effects. For example, if the tape tension is too high, the tape is liable to be damaged and magnetic heads may be abraided. On the other hand, if the tape tension is too low, the contact between the tape and the head becomes so loose as to make the quality of the reproducing signal bad.
A well-known arrangement for maintaining a constant tape tension in the reverse mode is to exclusively supply a high quality drive motor for rotating the S-reel and to detect the winding radius by certain sensor means so as to control the motor according to the result of the detection. While such arrangement has proven its effectiveness with regard to the tape tension, it has suffered the disadvantage of being complex in construction, difficult to manufacture and high in cost.